142 THE THEORY OF MUTATION 



best developed as a rule either shows the racial char- 

 acter in about half of the seedlings produced, or else 

 exhibits in the great majority of its members a com- 

 bination of the character of the species with that of 

 the race. As an example, we may take the case of 

 variegated plants, in which the leaves show streaks 

 or patches of a yellow colour owing to the want of 

 development of the proper green tint. An ordinary 

 variegated plant, then, is looked upon as showing a 

 combination of the green type with the yellow char- 

 acter of a completely modified race the aurea variety, 

 although the latter exists as such only in a few rare 

 cases, in which the plants bear leaves showing no 

 green pigment at all. On the other hand, many 

 species of plants produce a small proportion of varie- 

 gated individuals at each sowing, as is often the case, 

 for example, with Indian corn ; and this circumstance, 

 according to de Vries, indicates the existence of the 

 corresponding half-race. 



The relative development of the two coexisting 

 characters in such cases is highly variable, as anyone 

 may observe for himself in variegated grasses and 

 similar plants. 



It might be supposed that it would be possible to 

 pass from the species to the half-race, thence to the 

 mid-race, and so on to the complete race simply by 

 selection. De Vries shows that this is very rarely, if 

 ever, the case. He regards the passage from a half- 

 race to a mid-race, for example, as a mutation, and his 

 observations seem to show that this transition is not 

 more frequent than any other mutations. 



